The present invention relates to a device for determining the five linearly independent components of a gradient tensor of a magnetic field.
For measuring completely the components of a magnetic gradient tensor it is required in principle that spatial changes in the magnetic field have to be measured in the direction of the areal normal or right angle axis of a transducer or receiver coil that is to be responsive to a magnetic field. For this it has been proposed to use three dimensionally arranged and constructed coil arrangements and devices. For example for measuring the gradient tensor wound wire coils are used as shown e.g. by M. W. Wynn et al published in IEEE Trans Mag, Mag-11.2, 1975 or a three dimensionally coating technology is used as shown e.g. in German patent 32 47 543 A1. The calibration of the magnetometer is typically carried out by means of auxiliary devices as shown in the Wynn et al publication above, or numerically or electronically as shown e.g. by P. V. Czipott, and W. N. Podney published in IEEE Transactions Mag, Mag-25.2 1989.
Wound coils usually exhibit, in principle, relatively large errors. On the other hand planar coils can indeed be manufactured fairly simply in thin layer configurations. However, these configurations are sufficient only for measuring the nondiagonal elements of the gradient tensor. That means one can arrange two coils in the x-y plane to determine the gradient dBz/dx and dBz/dy. For the gradient dBz/dz the coils have to be arranged in two planes, one above the other. U.S. Pat. No. 4,646,025 describes such a device for measuring the diagonal elements; two coils are arranged in different planes one above the other. Two coils are arranged in the same plane for measuring the nondiagonal elements. Particularly the aforementioned coil arrangement, wherein one coil is placed above the other one, is very difficult to realize through planar elements. The thin layer technology on the other hand is very difficult to practice for any arrangement in three dimensional structures, including e.g. to extend coatings across edges or the like.